In on-demand ink jet printing, a grid of pixel locations is defined on a print media surface. During the print process, each pixel location may receive a droplet of ink from a set of ink ejection nozzles on a print head as the print head passes horizontally over the print media surface. In many systems, the pixel grid may be considered to comprise a series of vertical columns of pixel positions, and the ejection nozzles are also functionally organized as a vertical column. The vertical spacing between nozzles corresponds to the vertical pixel spacing, which will typically be approximately 50 to 600 pixels per inch, resulting in a vertical inter-nozzle spacing of about 40 to 500 microns. As the vertical column of nozzles passes over each vertical column of pixel locations, the appropriate droplets are deposited. In some printers, known as xe2x80x9cpage widexe2x80x9d printers, the vertical column of nozzles spans one dimension of the image, and image generation is performed by incrementing the media beneath the print head in the orthogonal dimension. In most printer designs, however, the vertical column of nozzles is much shorter in length than the total number of pixels in a vertical pixel column of the whole image, and the nozzles are mounted on a moveable print carriage. In these printer embodiments, the print head is sequentially passed over one horizontally extending swath of the image at a time, and the media is incremented between each pass.
It has been found that the print quality of images produced by moving carriage printers is improved dramatically with the use of xe2x80x9cmulti-passxe2x80x9d print techniques. In these print methods, portions of the nozzle column pass over the same segment of media two, four, six, or even more times, with each pass laying down a fraction of the ink droplets required to complete a swath. This technique avoids several print quality problems that are associated with depositing a large amount of ink in a single pass of the print head.
Unfortunately, however, with the same print carriage speed, the increased quality of four or six pass print modes comes at the price of a four or six fold increase in print time. Unless the print carriage speed is increased, therefore, the additional passes result in a severe reduction of printer throughput.
The speed at which the print head travels is limited by two factors. The first is the fact that a given nozzle can only be fired at a pre-determined maximum rate. In a single pass mode, therefore, the carriage cannot travel over more pixel columns per second than the maximum firing rate of the nozzles. This limitation is not a significant concern with the use of multi-pass print modes, however, because the nozzles can be programmed to skip certain pixels on certain passes. In a two-pass print mode, for example, the carriage speed can be doubled over the single pass speed because it is possible to ensure that a given nozzle only prints on at most every other pixel location as it passes over the pixel columns.
Although it first appears that increasing the number of passes in a multi-pass print mode would always allow a corresponding increase in print head speed, this is not the case because of a second factor which limits print head speed. This factor is the additional minimum time required to deposit ink onto any individual one of the pixel columns. One reason for this minimum time is that in most print head designs, the vertical column of nozzles is actually arranged as a horizontally spaced series of sub-columns. These sub-column arrangements are used because the simultaneous firing of too many nozzles, especially adjacent nozzles, is undesirable. In thermally activated print heads, for example, the firing of too many nozzles simultaneously results in a large power dissipation which is expensive to supply and which causes an excessive temperature increase in the print head. In addition, in both thermally and piezoelectrically actuated print heads, the firing of one or a set of nozzles may cause droplet volume and velocity changes or may otherwise interfere with the firing of other nozzles of the print head.
In one commercially available thermal print head from Lexmark Corp., for example, a nozzle array of 208 nozzles is divided into two separate columns of 104 nozzles, with each of the two columns being further arranges as 13 horizontally adjacent sub-columns of 8 nozzles each. As a column of nozzles passes over a pixel column, the 13 sub-columns are enabled sequentially as they become approximately centrally positioned over the pixel column. With this print head, each nozzle requires about 3 micro seconds to deposit an ink droplet. The nozzle column must therefore remain over each pixel column for at least about 13xc3x973=39 microseconds. If the pixel columns are 1/600 inches wide (i.e., the printer is a 600 dpi printer), the maximum speed of the print head over the media is about 40 inches per second. It has therefore been found that carriage speed in four, six, or eight pass print modes, for example, cannot be increased significantly above what is possible in two pass mode. The increase quality provided by the additional passes results in a proportional increase in the time it takes to print an image. Increases in carriage speed are thus highly desirable, as they would allow improvement in printer throughput without sacrificing print quality.
In one embodiment, a method of increasing the speed of multi-pass printing onto a series of adjacent pixel columns with an ink jet print head is provided. The ink jet print head comprises a plurality of sub-columns of ink ejection nozzles actuated by a corresponding plurality of address lines, and the method comprises asserting only a pre-defined subset of the address lines within each of the pixel columns.
Embodiments of ink jet printers are also provided. In one advantageous embodiment, an ink jet printer comprises processing and logic circuitry for controlling ink ejection by selectively asserting address and power lines of an ink jet print head in accordance with print data and a print mask. The printer further comprises a memory coupled to the processing and logic circuitry for storing the print mask. The print mask defines a droplet deposition pattern for each nozzle which guarantees, regardless of print data content, that at most only approximately half of the address lines will be required to be asserted as the print head passes over each pixel column of the image being printed. This makes it unnecessary to move the print head slow enough to assert all of the address lines over each pixel column and thus allows for a higher print head speed over the print media.